Recent Comments

I did not attempt to print gears because of the small size- these very cheap ($2) injection molded gears are much better than you could ever print.

The reason the nuts turn in opposite directions is because the filament will try to twist with the nut if you use a single nut. Using two nuts turning in opposite directions cancels the twisting torque. You could use a single nut if you used some other method to prevent the filament from twisting with the nut.

Note that one nut is right hand threaded and the other is left hand threaded. That way when they turn in opposite directions the filament moves in one direction through both nuts.

Will you be posting the drive gears? Also wouldn't you be able to have just one gear pushing the filament if you made the "nut" longer somehow, but either taping a piece of stock to the right diameter or stacking a few nuts? Seems like that would also make the assembly smaller.

The nuts are 6-32, one is left hand threaded and the other is a normal right hand threaded nut. The right hand nut can be a normal off-the -shelf part, but you have to be careful to choose one that bites into 1.75mm filament. Some start with larger holes and will just slip over a piece of filament. When you make the left hand threaded nut using a left hand tap, just like the right hand threaded nut, you want to start with a hole size that will allow the nut to bite the filament once it is threaded. If you use too large a hole the nut will just slip. You want to try to macth the friction of the two nuts, so start with a small hole and then file the left hand threaded nut open a little at a time to check it.

One filament drive nut is left hand threaded, made by tapping mild steel with a left hand tap, the other is standard 6-32 nut. Make sure the nuts thread onto 3mm filament- some start with larger holes and will slip right over the filament.

Will you be posting the drive gears? Also wouldn't you be able to have just one gear pushing the filament if you made the "nut" longer somehow, but either taping a piece of stock to the right diameter or stacking a few nuts? Seems like that would also make the assembly smaller.

I did not attempt to print gears because of the small size- these very cheap ($2) injection molded gears are much better than you could ever print.

The reason the nuts turn in opposite directions is because the filament will try to twist with the nut if you use a single nut. Using two nuts turning in opposite directions cancels the twisting torque. You could use a single nut if you used some other method to prevent the filament from twisting with the nut.

Note that one nut is right hand threaded and the other is left hand threaded. That way when they turn in opposite directions the filament moves in one direction through both nuts.

The nuts are 6-32, one is left hand threaded and the other is a normal right hand threaded nut. The right hand nut can be a normal off-the -shelf part, but you have to be careful to choose one that bites into 1.75mm filament. Some start with larger holes and will just slip over a piece of filament. When you make the left hand threaded nut using a left hand tap, just like the right hand threaded nut, you want to start with a hole size that will allow the nut to bite the filament once it is threaded. If you use too large a hole the nut will just slip. You want to try to macth the friction of the two nuts, so start with a small hole and then file the left hand threaded nut open a little at a time to check it.

I just tested it with 30 tooth drive gear on the motor and 20 tooth gears on the nuts. This allows the assembly to be reduced in size and theoretically increases maximum print speed. It also increases the load on the motor, and recent test results indicate it increases the load too much. The NEMA-17 motor I am using doesn't seem to have adequate torque to push even one of the 20 tooth gears with the filament in the nut. I'm going to go back to the 30/30 gear device and see if I can get the retraction to work better. More testing this week...

There is a lot of work still to be done. In the video I have the nuts located in-board, so there is a pretty long distance from the entrance to the hot-end. Having the nuts in-board allows for very easy loading of the filament as you can see in the video. I have another version where both nuts are outboard, but had some trouble with those nuts not gripping right, so the one that worked is the one I am testing. Those nuts were made on lathe then tapped by hand. They fit over the ends of the tubes and lined up perfectly.

It remains to be seen how this design handles variations in filament diameter/cross section shape. I am looking into threading a collet from a dremel tool to serve as a nut with adjustable tension.

I think printed gears may not be strong enough for this application, but who knows? Metal gears would allow the size to be shrunk even further, but they tend to be very expensive. The gears I used are actually for 3/16" shaft but its so close to 5mm it works fine and they hang onto the motor and brass tubes tightly.

I really like this. Since the feed rate isn't dependent on the diameter of a hobbing (and the varying engagement of the
hobbing into the plastic), there's one less variable to account for when calibrating the printer.

Also, since there is no longer a need for a gap between the filament feed exit and the hot end, the slight chance of misfeeding/filament buckling can be eliminated. Well done!

A couple of thoughts (not critique, just curious/brain dumping):
Does the threading create any burr or swarf in the plastic?
I'm thinking the plastic should be soft enough to form rather than cut the threads. That would be ideal anyway, since any plastic debris buildup may cause problems, and would (probably) make calibration less accurate.

The gears look like they could be modelled with involute_gears.scad (from MCAD). It might make it easier to fit/center the nuts if you can customize the centers of the gears. Not sure how printed gears would work for bevel gears, but I think it's worth a try.

A couple of cross braces should take care of the wobble, but i suspect it might not even be a problem once the nuts are centered well.

I'll definitely try this approach once I get my current projects out of the way.